1. Field of the Invention
The present invention relates to a process for production of aromatic hydrocarbons and more particularly to a process for producing aromatic hydrocarbons from a naphtha fraction efficiently over a long period of time.
2. Description of Related Art
For petrochemical production of aromatic hydrocarbons such as benzene, toluene, xylene and the like, a method for catalytic reforming of a naphtha fraction with a strong acid zeolite catalyst, e.g., ZSM-5 has been known (see, for example, Japanese Patent Publication Nos. 42639/1981, 23368/1983, Japanese Patent Application Laid-Open Nos. 92717/1978, 140934/1981).
This method, however, has disadvantages in that cracked gases are formed in a large amount and therefore the yield of aromatic hydrocarbons is low.
In order to overcome the above problems, a method using a zeolite L catalyst with platinum deposited thereon has been proposed (see, for example, Japanese Patent Application Laid-Open No. 80333/1984). Although this method has an advantage that aromatic hydrocarbons can be produced with high selectivity, it suffers from problems that the catalytic activity is rapidly reduced and the catalyst cycle is short. To lengthen the catalyst cycle, it is necessary to maintain the yield of aromatic hydrocarbons at a low level such as below 50%.
In order to overcome the above prior art problems, the present inventors have made extensive investigations from various view points.
As a result, it has been found that cyclopentane or methylpentane contained in a naphtha fraction feed is responsible for the deposition of a large amount of coke becoming a catalyst poison for zeolite catalysts with large pore radius such as L-zeolite containing Group VIII metals of the Periodic Table, such as platinum and the like, and thus markedly shortens the catalyst cycle. Based on the findings, it has been found that if the cyclopentane content of the naphtha fraction feed is controlled to not more than 1% by weight or the methylpentane content is controlled to not more than 10% by weight by distillating the naphtha fraction feed or separation of high purity normal paraffins, the catalyst cycle (catalyst life) can be lengthened and aromatic hydrocarbons can be produced efficiently.